Photographic stripping films



July 25, 1961 A. w. GRUMBINE 93, 2

PHOTOGRAPHIC STRIPPING FILMS Filed Aug. 6, 1959 GELATIN+ STLVER HALIDE GELATIN GELATIN SUBLAYER CELLULOSE NITRATE MEMBRANE ETHYLCELLULOSE- CELLULOSE NITRATE CELLULOSE ACETATE CELLULOSE NITRATE GELATIN SUBLAYER GELATIN+ANT|HALAT|0N DYE FIG.1

INVENTOR ARTHUR WILLIAM GRUMBINE ATTORNEY United States Patent Ofice a illi i? 2,993,792 PHOTOGRAPHIC STRIPPING FILMS Arthur William Grumbine, Parlin, N.J., assiguor to E. L du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Aug. 6, 1959, Ser. No. 831,939 11 Claims. (CI. 9683) This invention relates to photographic films, and more particularly to photographic stripping films useful for making improved etching resists for photomechanical printing plates, and to processes for making such films.

In the photogravure field it has been the custom to use the so-called carbon tissues consisting of a pigmented gelatin layer on paper which is sensitized with potassium bichromate just prior to use. This system involves many disadvantages including instability of the sensitive tissue, inconvenience in handling and long exposure time. An improvement over the carbon tissue process was made with the advent of the stripping films of White et al. U.S. Patent 2,638,417 and the use of the processes set forth in that patent and in Boyer U.S. Patent 2,650,878. The photographic elements and processes described in these patents allow the technician not only to cut the time of making a gravure plate to a fraction of the time needed in the carbon tissue process but also allow the technician to determine the quality of the reproduction before the relief image is formed.

However, the stripping films of these patents have a tendency toward causing non-uniformity in the etching step. This is due to the fact that not all of the protective membrane and/or the gelatin sublayer or anchor layer is removed by the organic solvent solution after the exposed and processed film is squeegeed onto the copper plate, as described in Example I of U.S. Patent 2,650,878. The residual water-insoluble film remaining on the emulsion image-containing layer results in the formation of an unfaithful relief image after washing with warm water. Upon etching the metal plate by the usual etching solutions, the imperfections in the relief are carried into the etched image and into an ink print made therefrom. The complete removal of the colloid substratum or anchor layer and the organic solvent-soluble protective film membrane is diflicult to control in plate-making since both of these layers are thin and colorless and there is no practical way of detecting their presence until the actual etching has started and then it is too late to remove the deleterious residual material. Because of this, costly gravure printing plates and cylinders may be ruined.

Various methods have been proposed to assure complete removal of the protective membrane and the colloid sublayer or anchor layer, including the use of various single and mixed solvents, changes in thickness of the protective layer and by modifying the solvent action of the subbing solution, or by dry-stripping, etc. Another proposed method involves wiping of the softened gelatin with wet cotton during the hot water bath treatment in order to assist in the removal of the anchor layer skins. Even this is disadvantageous and frequently causes nonuniformity in thickness of the resist and breakthrough of the background area duringthe etching treatment. None of the proposed methods are dependable or sutficiently satisfactory for large scale operation.

An object of this invention is to provide an improved:

photographic stripping film. Another objectis to provide such a film which is useful in making high quality etched printing plates. Yet another object is to provide such a film which results in an etching relief that is clean and constitutes a faithful reproduction of the original picture or image. A further object is to'p'rovidefsuch a film which can be used successfully by theordinary technicians A still further object is to provide processes for making such films from readily available materials. A still further object is to provide such processes which can be carried out with the conventional coating equipment of the photographic manufacturer. Still further objects will be apparent from the following description of the invention.

The above objects are attained in accordance with the invention (1) by applying to the surface of a hydrophobic organic macromolecular polymer film or membrane a gelatin solution containing an organic solvent or mixture of solvents for said film base but chemically inert with respect to said gelatin whereby an extremely thin gelatin anchor layer is formed both in and on the surface of said membrane, (2) applying to the anchor layer an aqueous solution of gelatin (which is soluble in water at 30 C. to 40 C. to the extent of at least 10% by weight) to form a contiguous layer of unhardened gelatin and having a thickness from about 0.06 to about 0.35 mil and (3) applying to the surface of the unhardened gelatin layer a gelatino silver halide emulsion layer. The anchor layer, in general, will be 0.001 or less to 0.004 mil in thickness. Since the organic solvent etches the surface of the membrane and deposits the gelatin in and on its surface, it is diflicult to determine the thickness of the anchor layer. In general, the gelatin anchor layer has a coating weight of about 0.4 to about 0.7 mg. of dry gelatin per square decimeter. When the membrane has two or more strata, e.g., has a macromolecular polymer waterproofing layer on its surface, it is, of course, the latter layer which bears the gelatin anchor layer.

The film, or membrane, for the gelatin anchor layer of the stripping films of this invention generally are quite thin, e.g., from 0.08 to 0.3 mil in thickness and this membrane is laminated directly, or by means of a stripping layer, to a temporary or removable film base. The stripping layer preferably has fair adherence to the film base when the entire photographic film is wet or dry, but by the application of mechanical force, is readily strippable therefrom.

The preferred stripping films of the invention comprise a flexible, hydrophobic macromolecular organic polymer film base having a water-permeable organic colloid backing layer on one surface and on the other surface, in order, a dry-stripping layer, a thin water-insoluble membrane which is soluble in an organic solvent, especially a volatile solvent which is soluble in or miscible with water to a substantial extent, an anchor layer having a thickness from about 0.001 to about 0.004 mil in and on the surface of said film composed of gelatin which is soluble in water at 30-40 C., a contiguous unhardened layer from 0.06 to 0.35 mil in thickness composed of such soluble gelatin and a Water-permeable colloid silver halide emulsion layer.

The photographic silver halide emulsion layer of the foregoing elements preferably are markedly sensitive to blue and higher wave lengths of light, e.g., green and red light. Thus, the emulsion layer can be orthochromatic or panchromatic photographic emulsion layer, and if desired, the layer may be colored by means of a dye or pigment to improve the contrast of the relief on a metal surface made by exposure and processing of the film to form a resist.

With respect to the foregoing photographic film elements, the presence of the additional contiguous layer of soluble gelatin has the advantage in the preparation of resists that any residual sublayer skins and any undi'ssolved protective layer, or so-called permanent support,"fioat away with the contiguous soluble layer of gelatin during the hot water washing operation. Only the hardened relief image remains on the surface of the metal plate. This additional contiguous layer absorbs suificient water-during the laydown procedure that it becomes sufficiently softened when the assembly is placed in, or treated with, hot water to permit stripping either said layer or both said layer and the permanent support frlom the developed emulsion layer adherent to the metal p ate.

The film base can be composed of any polymeric material capable of forming flexible, self-supporting hydrophobic films. Many suitable such supports are known in the art. Thus, they may be composed of a cellulose derivative, e.g., cellulose acetate, cellulose propionate, cellulose acetate/propionate, cellulose acetate/butyrate or other simple or mixed carboxylic acid esters of cellulose; 21 superpolymer, e.g., a polyamide, for instance, polyhexamethylene adipamide; poly(vinyl chloride co vinyl acetate), polyvinyl acetals of aldehydes such as formaldehyde, acetaldehyde and butyraldehyde; polyalkylene terephthalates, e.g., polyethylene terephthalate. Films composed of such materials may have a thin water-proofing layer on one or both surfaces. Suitable such layers can be composed of vinylidene copolymers, e.g., vinylidene ehloride/isobutylene, vinylidene chloride/acrylonitrile, and vinylidene chloride/methacrylate/itaconic acid copolymers. The vinylidcne chloride copolymer coated polyalkylene terephthalate oriented films of Alles et al. US. Patent 2,627,088 are particularly useful supports.

The thin membrane or film used as the permanent support may be made of any film-forming material such as those described in the preceding paragraph.

With regard to the silver halide emulsion layer, the

light-sensitive salt may be silver chloride, silver bromide,

silver chlorobromide,'silver iodobromide, etc. It preferably contains a soft gelatin composition as the binding agent. That is, it should be capable of being later differentially hardened in a developing or bleaching bath so that a relief image can be formed by washing away unhardeued emulsion. In addition, the ratio of gelatin to silver halide is preferably high, i.e., a relatively large quantity of gelatin to a relatively small quantity of silver halide. A satisfactory range is from 2 to parts of gelatin to 1 part of silver halide.

A representative photographic film element of the invention is shown in the accompanying drawing which constitutes a part of this application. Referring to this drawing, FIG. I is a vertical, cross-sectional view of the film element in Example I.

In making resists from a film element of the invention, it is exposed by one of the usual manners to produce the desired latent image in the photosensitive layer. For instance, in making a gravure plate, the film is exposed through a screen for a suitable time and then through a continuous tone positive, or vice versa. ventional types of screens and sequences of exposure can be used, including the screens and procedures disclosed in Kott U.S. Patent 2,628,903 granted February 17, 1953. The exposed film is then developed in a hardening developer as described in Boyer et al. Patent 2,650,877 and 2,650,878 (particularly in col. 1, line 22 to line 11, col. 2), and in White et al. 2,638,417, and further treated and used to form an etched plate as disclosed in said patents.

The invention will be further illustrated but is not intended to be limited by the following examples wherein the photographic film is made in the substantial absence of actinic light and all parts and percentages are by weight, unless otherwise indicated.

Example I With reference to the drawing, 5 mil cellulose acetate film 1 is coated with a cellulose nitrate solution in acetone to form film 2, on which is coated a gelatin anchor layer 3 (coating weight 6.5 mg./dm. from a 1.1% solution in 57.3% methanol, 40% acetone, 0.4% salicylic acid and 1.2% water. On layer 3 there is coated an antihalation layer 4 comprising gelatin, chrome alum and Acid Magenta O (Rowes Colour Index No. 692). On the other surface of film 1 there is coated a 0.2-mil drystripping layer 5 from a solution containing 3.1% ethyl Other concellulose having an ethoxy content of about 47-49% by weight, 5.9% of a cellulose nitrate-methanol solution containing 23.5% by Weight of cellulose nitrate of about 11.75% to about 11.99% nitrogen content, 20% butyl alcohol, and the balance acetone. On layer 5 there is coated a 0.2-mil cellulose nitrate membrane 6 from an acetone solution, and next is coated a gelatin anchor layer 7 at a coating weight of 0.55 mg./dm. and having a thickness of about 0.002 mil from the following solution:

Parts Gelatin 0.30 Salicylic acid 0.15 Water 2.40 Methyl alcohol 0.60 Ethyl alcohol 76.55 Acetone 20.00 Next there is coated an unhardened, soluble gelatin layer 8 about 0.16 mil in thickness and a coating weight of about 50 nag/din. based on the weight of dry gelatin from a solution of the following composition:

Percent Gelatin 5.500 Potassium chloride .032 Saponin .045 Glycerine .158 Ethyl alcohol 1.820 Thymol .010 Water to total 100 Over layer 8 there was coated an orthochromatically sensitized gelatin chlorobromide emulsion containing 25 mole percent of silver chloride. The coating weight of the resulting emulsion layer 9 was about 62 mg. of silver halide per square decimeter which gave a layer thickness of about 0.6 mil.

The resulting dried film structure was exposed-1n a vacuum printing frame as follows:

Half of the area of a sheet of the film was exposed through a ISO-line conventional gravure screen for 15 seconds to a 10-watt tungsten lamp placed 8 feet from such screen. The entire sheet was then exposed in a step wedge manner, that is, both the screened and unscreened areas were exposed in a series of exposures from zero to 40 seconds to a 15-watt tungsten lamp at 8 feet to'produce a wide range of silver densities.

The film was then processed as follows:

(1) Immersed for 3 minutes at 68 F. in a solution of the following composition:

Sodium metabisulfite grams 5 Sodium sulfite do 10 Metol do 1 Hydroquinone do 2 Pyrogallol ..do 6 Potassium bromide do 3 Anionic wetting agent (Eumercin NB 10% aqueous) 1Tll. 2

Water to make 1000 ml.

(2) Immersed for 2 minutes at 68 F. in a solution of the following composition:

Sodium carbonate (monohydrate) grarns 100 Anionic wetting agent (EumercinNB 10% aqueous) 7 ml 2 Water to make 1000 ml.

(3) Washed for 1 minute in water 68 F- (4) Fixed for 3 minutes at 68 F. in a solution hav ing the following composition: Sodium thiosulfate "grams-.. 200 Borac acid do.. 15

Water to make 1000 ml. 7

(5) Washed for 10 minutes in water at 68 F.

(6) Thefilmwas thendried.

After drying, the film was placed in a tray of water at 120 F. to determine at what density the exposed emulsion fuses onto the substratum.

Also at this time, as a comparative test, a second film, made according to Example I of U.S. Patent 2,638,417 and having the structure disclosed therein, was exposed and processed in the manner described above. The area of the emulsion of the second film which was not exposed to the 150-line screen floated away from the substratum and base in the form of a hardened skin up to a density of 2.84. From that point on this area of the emulsion layer was very tightly fused to the sub and base. The whole area which had received both the screen and wedge exposures was fused tightly even where only the screen alone had been exposed.

In the first film having the clear gelatin layer 8, the emulsion layer released up to a density of 400+, both in the screened and unscreened areas, thus exhibiting marked improvement in the release of the emulsion layer due to the presence of the extra soft gelatin layer.

Example II A stripping film having the structure of FIG. 1 in Example I but having a coating weight of 60 milligrams per square decimeter in layer 8 and a photographic emulsion coating weight of 71 milligrams per square decimeter in layer 9 was made in like manner. The resulting photographic film was exposed in a step wedge manner and processed according to the procedures described in Example I. The exposed film, when immersed in a tray of water at 125 F., released the emulsion layer up to a density of 4.00 where it was exposed to the tone image only, and up to a density of 3.79 in the area where it was exposed to both the gravure screen and continuous tone image.

Example III A stripping film having the structure of FIG. 1 in Example I but having a coating weight of 25 milligrams per square decimeter for layer 8 was made after the manner described in that example. Layer 8 was coated from a solution having the following composition:

The gelatin silver halide emulsion having the same composition as that in Example I was coated to a coating weight of 65 mg. per square decimeter.

The resulting photographic element was exposed and processed as described in Example I. Upon immersion in a water bath'having a temperature of 125 F., the emulsion was released up to a density of 3.52 where the film was exposed to a tone image only, and the emulsion layer released up to a density of 3.49 where the film was exposed to both the gravure screen and tone images. These results may be compared to the normal film of Example I of U.S. Patent 2,638,417 which released up to a density of 2.84 where it was exposed to a tone image only and did not release at all Where it was exposed to a conventional gravure screen and the tone image.

Example IV A film structure as described in Example I and illustrated by FIG. 1 was exposed, processed and etched along with a control film having the structure set forth in Example I of U.S. Patent 2,638,417 in the following manner:

1 Difference in exposure to compensate for minor speed variation.

Processing: Both films were processed as follows:

' DEVELOPMENT IN A TWO BATH HARDENING DEVELOPER SOLUTION Asolution (3 minutes) Sodium metabisulfite ..grams.. 5 Pyrogallol do 5 Eumercin NB 10% aqueous ml 2 Water to make 1 liter.

B solution (2 minutes) Sodium carbonate (monohydrated) ..gr-ams Eumercin NB 10% aqueous rnl 2 Water to make 1 liter.

Processing continued as described in Example I.

The processed films were then adhered, emulsion side to the same sheet of a gravure copper plate, by standard procedure as practiced in the art for laying down gravure resist material. The heavy clear base supports as shown in FIG. 1 and Example I of U.S. 2,638,417 were drystripped from the films. The nitrate membrane protec. tive layer was dissolved away by successive applications of a solution composed of equal parts of diacetone alcohol and isopropyl alcohol and not over 0.5% water.

' The copper plate containing both films was then immersed in a bath ,of water at F. for ten minutes to dissolve away all unexposed, unhardened gelatin; thus forming a relief image of varying thickness of hardened gelatin on the copper, inrelation to the silver densities. After thorough drying of the gelatin relief image and covering over the bare areas of copper with asphaltum as is common practice in the art, the step wedge relief images were etched into the copper. Normal gravure etching procedure was used, by employing ferric chloride etching solutions of 45, 43, 42, and 41 Baum respectively. Bot-h step wedge relief images were etched simultaneously for the same period of time. The following table indicates the etching times for each density of the step wedge.

Density in Time Density in Time Density of Steps of (Minutes) Steps of (Minutes) Positive Wedge of Etching Wedge of Etching Wedge test Started in Control Started in Steps Steps 1.80 .74 1% .71 1. 66 75 2 73 l l. 52 .78 2% .77 1% 1. 38, .80 4 .83 4% 1. 22 .87 7% .92 8% 1. 06 .96 10% 1.03 11% 91 1.11 13% 1. 20 1.4% 77 1. 28 16% 1. 38 17 .63 1. 54 19 1.63 19 .47 1. 80 23 1. 83 22 32 2. 11 25% l. 98 23% 18 2. 37 2.10 25% .04 2. 52 2. 21 26 The average usable density range of a gravure positive extends from 0.40 to 1.60. It will be noted that etching times for the steps within these limits for both the test film and the control are very similar. Approaching the end of this range the test film slowed up in etching because of the higher shoulder in the characteristic H and D curve which is evidenced by the range of densities set forth above. This provides added protection against breakthrough of the background and better separation of the highlights.

Particularly useful stripping layers are the ethyl cellulose/cellulose nitrate stripping layers containing 6 to 9 parts of the former to 4 to 1 parts, by weight, disclosed in White et al. U.S. Patent 2,638,417. The ethoxy content of these others can vary from 45 to 50% and the nitrogen content of the cellulose nitrate from about 10.5% to about 14.0%. The layers may vary in thickness and 0.08 to 0.3 mils being a practical range.

While the invention is primarily directed to non-dyed emulsion and clear soluble gelatin layers, dyes may be incorporated in one or both layers for various reasons. For example, the auxiliary gelatin layer may contain a non-halation dye and thus make it unnecessary to have a gelatin non-halation layer on the back of the temporary support.

Various photographic sensitizing dyes can be added to the emulsions if desired to extend their sensitivity to the green and red regions of the visible spectrum. There are a large number of such dyes known and available and they include the simple cyanine, carbocyanine, pseudocyanine, kryptocyanine, merocyanine and styryl dyes, etc.

The advantages of the structure of the invention may be summarized as follows:

(1) Complete removal of the substratum and any remaining protective membrane is assured.

(2) 'Shouldering of the characteristic sensitometric curve is minimized within the usable portion of the curve without substantially aifecting the remainder of the tones. This, of course, improves the separation of the highlight tones.

(3) Better protection against breakthrough of the background of the resist during etching is provided by the high shoulder of the characteristic curve.

(4) The necessity for wiping the resist during the hot Water bath, which contributes to the non-uniformity in etching, is eliminated.

(5) A simplified structure without a protective membrane is also possible when using an auxiliary soft clear gelatin layer.

This application is a continuation-in-part of application Serial No. 764,374 filed Sept. 30, 1958.

What is claimed is:

1. A photographic film element comprising a thin flexible hydrophobic membrane composed of a macro molecular organic polymer soluble in an organic solvent and bearing, in sequential direct contact, a gelatin anchor layer not more than .004 mil in thickness etched into and on a surface of the membrane, a contiguous unhardened gelatin layer from 0.06 to 0.35 mil in thickness on said anchor layer and a light-sensitive, waterpermeable organic colloid-silver halide emulsion layer on said contiguous layer, the gelatin of said contiguous layer being soluble in water at 40 C. to the extent of at least by weight, the other surface of said flexible membrane being laminated to a second hydrophobic flexible film base by means of a stripping layer.

2. An element as set forth in claim 1 wherein the contiguous gelatin layer is applied from essentially aqueous solution.

3. A photographic element as set forth in claim 1 wherein said flexible membrane is composed of cellulose nitrate. i

4. A'photographic film element as set forth in claim 1 wherein said flexible membrane is adhesively joined to the film base bymeans of a dry-stripping layer composed of a mixture of a water-insoluble ethylcellulose and cellulose nitrate containing 6 to 9 parts of the former to 4 to 1 parts of the latter, by weight.

5. A photographic film as set forth in claim 4 wherein said membrane is 0.08 to 0.3 mil in thickness and said emulsion layer is an orthochromatically sensitized gelatino silver chlorobromide emulsion.

6. A photographic stripping film comprising a hydrophobic cellulose carboxylic acid ester film base having one surface coated, in sequential direct contact, with a thin moisture-proofing layer and a Water-permeable colloid backing layer and the other surface of the base coated, in sequential direct contact, with a stripping layer composed of water-insoluble ethyl cellulose and cellulose nitrate in the proportion of 6 to 9 parts of the former to 4 to 1 parts of the latter, by weight, a thin flexible membrane composed of a Water-insoluble film-forming material selected from the group consisting of cellulose esters, polyvinyl esters and polyvinyl acetals, a gelatin anchor layer not more than about 0.004 mil in thickness etched into and on the surface of the membrane, a con tiguous gelatin layer from 0.06 to 0.35 mil in thickness applied from essentially aqueous solution and a lightsensitive, water-permeable organic colloid-silver halide emulsion layer, the gelatin of said contiguous layer being soluble in water at 40 C. to the extent of at least 10% by weight.

7. A photographic element as set forth in claim 6 having an antihalation layer on said backing layer.

8. A photographic element as set forth in claim 6 wherein said ester is cellulose acetate and said film-forming material is cellulose nitrate.

9. A photographic element as set forth in claim 6 wherein said colloid of the silver halide emulsion layer is gelatin. I

10. The process which comprises applying to the surface of a thin flexible macromolecular organic polymer membrane which is soluble in an organic solvent, a dispersion of gelatin in an organic solvent for said membrane to form in and on the said surface an anchor layer composed of gelatin and having a thickness not more than 0.004 mil, drying said anchor layer, applying to said layer an aqueous solution of gelatin to form a contiguous layer of unhardened gelatin having a thickness from 0.06 to 0.35 mil and coating a gelatino silver halide emulsion layer on said contiguous layer.

11. A process as set forth in claim 10 wherein said membrane is composed of nitrocellulose. 1

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PHOTOGRAPHIC FILM ELEMENT COMPRISING A THIN FLEXIBLE HYDROPHOBIC ELEMENT COMPOSED OF A MACROMOLECULAR ORGANIC POLYMER SOLUBLE IN AN ORGANIC SOLVENT AND BEARING, A SEQUENTIAL DIRECT CONTACT, A GELATIN ANCHOR LAYER NOT MORE THAN .004 MIL IN THICKNESS ETCHED INTO AND ON A SURFACE OF THE MEMBRANE, A CONTIGUOUS UNHARDENED GELATIN LAYER FROM 0.06 TO 0.35 MIL IN THICKNESS ON SAID ANCHOR LAYER AND A LIGHT-SENSITIVE, WATERPERMEABLE ORGANIC COLLOID-SILVER HALIDE EMULSION LAYER ON SAID CONTIGUOUS LAYER, THE GELATIN OF SAID CONTIGUOUS LAYER BEING SOLUBLE IN WATER AT 40* C. TO THE EXTENT OF AT LEAST 10% BY WEIGHT, THE OTHER SURFACE OF SAID FLEXIBLE MEMBRANE BEING LAMINATED TO A SECOND HYDROPHOBIC FLEXIBLE FOLM BASE BY MEANS OF A STRIPPING LAYER. 